1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus, and more particularly to an image forming method and an image forming apparatus in which an image is formed on an image formation body by using an ink and an aggregation treatment agent.
2. Description of the Related Art
In the inkjet recording system, the recording is performed by discharging ink droplets from nozzles and causing the ink droplets to adhere to a recording medium or the like. By contrast with other systems, noise during the recording operation is low, running cost is low, and image recording with high resolution and high quality can be performed. The ink ejection system can be a piezoelectric system using the displacement of a piezoelectric element or a thermal system using thermal energy generated by a heat-generating element.
Image forming methods in the inkjet recording system generally can be classified into two systems: a direct image formation system in which an image is directly formed on a recording medium (for example, a mildly permeable medium such as coated paper for printing) and an intermediate transfer system in which an image is formed on an impermeable medium such as a plastic sheet and then the image is transferred onto a recording medium.
However, the problem associated with the inkjet recording system is that where dots are formed to a high density such that the adjacent dots overlap by using ink droplets that are continuously deposited onto the recording medium, the so-called landing interference (bleeding) occurs, namely, the ink droplets forming the adjacent dots merge under the effect of surface tension on the recording medium and dots of desired shape and size cannot be formed. Where the aforementioned landing interference occurs, when the dots have the same color, the shape of dots collapses, and when the dots of different colors are obtained, not only the dot shape collapses, but also colors are mixed.
A two-liquid aggregation system using a treatment liquid that reacts with ink and causes the ink to aggregate has been suggested as means for preventing such landing interference occurring between the ink droplets (dots) on the recording medium. For example, Japanese Patent Application Publication No. 2004-010633 discloses a technology that improves optical density, oozing, oozing between colors (bleeding), and drying time in the two-liquid aggregation system by imparting acidic properties to one liquid from among the liquid composition (treatment liquid) and ink and imparting alkaline properties to the other and controlling the aggregation ability of the pigment on the recording medium.
Further, an intermediate transfer system in which an image formed on an intermediate transfer body is transferred onto a recording medium has been suggested as a technique for preventing image deterioration caused by a liquid solvent (ink solvent etc.) present on the recording medium and curing of the recording medium. For example, Japanese Patent Application Publication No. 11-188858 discloses a method by which a powder layer (water-soluble resin) that demonstrates solubility or swellability with respect to ink droplets, can increase the viscosity of ink droplets, and can be peeled off from an intermediate transfer body is formed in advance on the intermediate transfer body before the ink droplets land on the intermediate transfer body, whereby high-speed recording is enabled, without bleeding into the permeable recording medium.
In the case of the above-described intermediate transfer method, a medium that is impermeable to inks (impermeable medium) is typically employed for the intermediate transfer body with the object of improving transferability. As a result, where ink droplets are continuously deposited so that the adjacent droplets (dots) overlap on the intermediate transfer body, a problem of bleeding (landing interference) is encountered, namely, the adjacent ink droplets merge on the intermediate transfer body under the effect of surface tension of the droplets and the desired dots cannot be formed. Such bleeding makes it difficult to perform high-speed printing.
The following methods have been suggested to resolve the above-described problems.
(1) A method of incorporating a two-liquid aggregation system using ink aggregation induced by a treatment agent into the intermediate transfer system. Japanese Patent Application Publication Nos. 2004-090595, 2004-114675, and 2005-170036 disclose methods that resolve the bleeding problem by applying an ink aggregating agent to the intermediate transfer body before the ink is deposited, thereby decreasing the flowability of the ink. In this case, a water-soluble pigment ink is used as the ink and a polyvalent metal salt or an acidic solution is used as the ink aggregating agent.
(2) A method of forming concavities and convexities on the intermediate transfer body
Japanese Patent Application Publication Nos. 2006-137127 and 2007-069584 disclose methods for forming concavities and convexities on the surface of intermediate transfer body and preventing bleeding by an anchor effect.
(3) Method using water-absorbing particles
Japanese Patent Application Publication Nos. 2000-280460 discloses a method for preventing bleeding by forming a layer composed of particles that are soluble in water or swell when water is supplied thereto on an intermediate transfer body and causing the particles to absorb the ink.
However, the below-described unresolved problems are still associated with image formation in the two-liquid aggregation system and intermediate transfer system, and further improvements are needed.
<Problems Arising in Two-Liquid Aggregation System>
Where an ink is deposited after applying a treatment liquid (aggregation treatment liquid) to an impermeable medium (intermediate transfer system) such as a plastic sheet or mildly permeable medium (direct image forming system) such as coated paper for printing, ink aggregates (coloring material aggregates) formed due to mixing and reaction (aggregation reaction) of the treatment liquid and ink do not remain in the desired position and move. A new problem that results therefrom is that the output image is greatly distorted with respect to the desired image.
A general process implemented in the two-liquid aggregation system in the related art will be described below with reference to FIGS. 40A to 40E. First, an aggregation treatment layer (liquid layer) 902 having a predetermined thickness is formed on a recording medium (impermeable medium) 900 having impermeability (see FIG. 40A). Then, ink droplets 904 are deposited onto the recording medium 900 having the aggregation treatment layer 902 formed thereon. Where the ink droplets 904 land on the aggregation treatment layer 902 located on the recording medium 900 (FIG. 40B), an aggregation reaction instantaneously starts on the entire surface of the ink droplets 904, the aggregation reaction advances before the ink droplets 904 reach the recording medium 900, and ink aggregates (coloring material aggregates) 906 are formed (FIG. 40C). Therefore, the ink aggregates 906 do not come into contact with the recording medium 900 or come into extremely weak contact with the recording medium 900 (FIG. 40D). As a result, the adhesion force (bonding force) between the ink aggregates 906 and recording medium 900 is insufficient, dots composed of the ink aggregates 906 that are in a rolling state assume an unstable adhesion state (for example, a state in which they float in the aggregation treatment layer 902), and cause image deterioration induced by the movement of coloring material.
Further, it has been found that when a liquid solvent (solvent component of the ink and aggregation treatment liquid) 908 on the recording medium 900 is attempted to be absorbed and removed by using, for example, an absorbing body 910 such as a solvent-absorbing roller composed on the surface thereof of a porous material, as shown in FIG. 40E, because the adhesion force between the ink aggregates 906 and recording medium 900 is insufficient, as described hereinabove, part of the coloring material adheres to the absorbing body 910. The problems that result from this drawback include image deterioration caused by insufficient amount of coloring material on the recording medium 900 and reverse transfer of the coloring material that has adhered to the absorbing body 910 to the recording medium 900.
In addition, the following problems (A)-(C) are associated with the method described in Japanese Patent Application Publication No. 11-188858.
(A) Because the coloring material present in the ink is not actively aggregated, where the ink droplets are ejected with a high speed not higher than 10 kHz, the swelling or thickening processes do not proceed in time and landing interference of the adjacent ink droplets occurs.
(B) Because the transferred image formation layer swells due to the absorption of ink solvent, the thickness of the image portion increases causing the so-called “pile height” problem. Problems arising when the image thickness increases include not only the image quality problem associated with change in appearance of the boundary of the printing zone and non-printing zone, but also a problem associated with that a step is felt when this portion is touched.
(C) Because the ink solvent is absorbed by the transferred image formation layer, the ink solvent oozes to the paper surface after transferring and the paper is deformed (the so-called “cockling”).
The aforementioned problems (B), (C) arise because an image is formed on the recording medium (paper), while the ink solvent is still contained therein. The problems (A) to (C) cause crucial quality deterioration in high-quality printing.
<Problems Associated with Intermediate Transfer System>
The following problems are associated with the related art of the intermediate transfer system discussed in sections (1) to (3) above that are described in relation to background art.
With the “method using ink aggregation induced by a treatment agent” of section (1), the tests conducted by the inventors demonstrated that when an ink and an aggregating agent react to form an image, shrinking of the image is caused by the ink aggregation reaction and image surface area decreases with respect to the intended one, thereby making it impossible to form a quality image.
Problems arising when the image forming method based on two-liquid aggregation is applied to the intermediate transfer system will be described below in greater details.
FIGS. 41A to 41D are a schematic diagram illustrating the behavior of the ink droplets (dots) in the related art when they land on an intermediate transfer body. FIG. 41A shows a state in which an aggregation treatment agent layer 912 composed of an ink aggregating agent (aggregation treatment liquid) is formed on an intermediate transfer body 910 (impermeable body), FIG. 41B shows a state of the aggregation treatment agent layer 912 before the ink droplets 914 land on the intermediate transfer body 910, FIG. 41C shows a state immediately before the ink droplet 914 lands, and FIG. 41D shows a state about 1 sec after the ink droplet 914 has landed. First, an aggregation treatment agent is applied to the intermediate transfer body 910 and the aggregation treatment agent layer 912 of a predetermined thickness is formed (FIG. 41A). Where an ink droplet 914 ejected from an inkjet head (not shown in the drawing) thereafter reaches the aggregation treatment agent layer 912, an aggregation reaction starts from the portion of the aggregation treatment agent layer 912 reached by the ink droplet, the viscosity of ink droplet locally rises, and an ink aggregate (coloring agent aggregate) 916 is formed (FIG. 41B). Immediately after the ink droplet 914 has landed on the intermediate transfer body 910, the ink droplet 914 spreads to a certain size on the intermediate transfer body 910 due to kinetic energy of the droplet, the aggregation reaction advances in the entire ink droplet 914, and viscosity rises (FIG. 41C). Where the aggregation reaction further proceeds as the time elapses, the ink aggregate shrinks as the solvent located in the ink droplet 914 is released to the outside (FIG. 41D). In this case, the dot size reduction occurs.
FIGS. 42A and 42B are a schematic diagram illustrating the behavior observed when an image is formed according to the related art. FIG. 42A shows the state immediately before the image is formed. In this case, a portion (image portion) 920 where a coloring material is to be applied and a white background portion (image-free portion) 922 are formed in accordance with the input image data. It will be assumed that the image portion 920 is formed by a plurality of dots. FIG. 42B represents a state about 1 sec after the image has been formed. The aggregation reaction proceeds similarly to the process illustrated by FIGS. 41A to 41D, shrinkage of the image portion 920 occurs, and the surface area of the white background portion 922 becomes larger than the desired one.
With the “method for forming concavities and convexities on the intermediate transfer body” of section (2) above, where concavities and convexities are formed on the intermediate transfer body surface, the contact surface area of the ink layer and intermediate transfer body increases and the adhesive force of the two becomes too strong. Furthermore, adhesion of the recording medium and intermediate transfer body decreases. The resultant problem is that transferability onto the recording medium is degraded. Further, the intermediate transfer body has to be cleaned after the transfer, but cleaning ability of the intermediate transfer body surface is degraded by the concavities and convexities.
FIGS. 43A and 43B show schematically the behavior in the transfer process in the related art. FIG. 43A illustrates how an image formed on the intermediate transfer body 930 having concavities and convexities formed on the surface thereof is transferred onto the recording medium 932, while a pressure is applied by a transfer roller (not shown in the drawing). The reference numeral 934 represents an ink layer configuring an image formed on the intermediate transfer body 930. FIG. 43B shows a state after the recording medium 932 has been peeled off (that is, the state after transferring). Where concavities and convexities are formed on the surface of the intermediate transfer body 930, the contact surface area of the ink layer 934 and intermediate transfer body 930 increases, as described hereinabove. Therefore, part of the ink layer 934 remains on the intermediate transfer body 930 after the transfer, thereby causing transfer defects such as image deterioration.
Further, in the “system using water-absorbing particles” of section (3) above, a particulate layer that has absorbed ink is directly transferred onto the paper. The resultant problem is that a thick image is formed on the paper. For example, when a standard inkjet ink with a pigment concentration in the ink of 4 parts by weight is used, the amount of ink with a thickness about 10 μm is required for high-concentration portions of the image, but when the image on paper has a thickness of 10 μm, a strong unpleasant feel is created by the appearance of the printed matter and problems are associated with quality thereof.
Thus, there is still room for improvement in the two-liquid aggregation system and intermediate transfer system.